Eastern Alps) 172-185 © Österreichische Geologische Gesellschaft/Austria; Download Unter Und

ZOBODAT - www.zobodat.at

Zoologisch-Botanische Datenbank/Zoological-Botanical Database

Digitale Literatur/Digital Literature

Zeitschrift/Journal: Austrian Journal of Earth Sciences

Jahr/Year: 2015

Band/Volume: 108_1

Autor(en)/Author(s): Pfingstl Stefan, Kurz Walter, Schuster Ralf, Hauzenberger Christoph A.

Artikel/Article: Geochronological constraints on the exhumation of the Austroalpine Seckau Nappe (Eastern Alps) 172-185 © Österreichische Geologische Gesellschaft/Austria; download unter www.geol-ges.at/ und www.zobodat.at

Austrian Journal of Earth Sciences Volume 108/1 Vienna 2015 Geochronological constraints on the exhumation of the Austroalpine Seckau Nappe (Eastern Alps)______

Stefan PFINGSTL1), Walter KURZ1)*), Ralf SCHUSTER2) & Christoph HAUZENBERGER1) 1) Institut für Erdwissenschaften, Universität Graz, Heinrichstrasse 26 / Universitätsplatz 2, A- 8010 Graz; DOI: 10.17738/ajes.2015.0011 1) Member of NAWI Graz (an InterUniversity cooperation between the University of Graz and the Graz KEYWORDS

1) University of Technology), Austria; Silvretta-Seckau Nappe System 2) Geologische Bundesanstalt, Neulinggasse 38, 1030 Wien, Austria; Austroalpine Unit cooling history *) Corresponding author, [email protected] geochronology

Abstract New Rb-Sr biotite age data from meta-granitoids of the Seckau Nappe (Eastern Alps) are discussed to constrain its tectonometa- morphic evolution. Within the basement of the Seckau Nappe a clear distinction between S- and I- Type granitoids can be establi- shed. The S- type granites are mainly part of the structurally uppermost sections and are covered by Permian to Late Triassic metasedimentary sequences of the Rannach-Formation. Within the AFM diagram all granitoids are characterized by a calcalkaline trend. This suggests formation of the melts during a subduction process. The granitoids are related to both pre-plate collision, syn-collision and post-collision uplift settings.______The Rb-Sr biotite age data (biotite - whole rock isochrons) range from 76 to 86 Ma, with eight samples showing ages around 84 to 86 Ma. The Rb-Sr biotite-ages are interpreted to date cooling below 300±50°C. Therefore the investigated part of the Seckau Nappe cooled down below ~300°C at about 85 Ma in the Santonian.______Referring to microstructural observations, the contact between the basement of the Seckau Complex and the Rannach-Formation was strongly overprinted by the formation of distinct shear zones that are characterised by extensional fabrics. Kinematics is characterized by conjugate sets of top-to-the-WNW and top-to-the-ESE. As feldspar is deformed by cataclastic deformation mechanisms, whereas quartz shows deformation mechanisms of dislocation creep and bulging dynamic recrystallization we assume that the conditions of deformation along the Seckau Complex with the Rannach-Formation are in the range of 300°C to 400°C. A comprehensive interpretation of the Rb-Sr biotite age data combined with deformation structures suggests that extension-reated exhumation of the Seckau complex occurred around 85 to 80 Ma.______

Neue Rb-Sr Biotitaltersdaten aus Metagranitoiden der Seckau-Decke innerhalb der Seckauer Tauern (Ostalpen) werden im Zu- sammenhang mit der tektonometamorphen Entwicklung dieser Einheit diskutiert. Innerhalb des Basements der Seckau-Decke kön- nen I- und S- Typ Granitoide klar unterschieden werden. Die S-Typ Granitoide sind mehrheitlich in den struturell höheren Abschnitten vorhanden und werden von den permisch bis untertriassischen Metasedimenten der Rannach-Formation überlagert. Im AFM Dia- gramm zeigen alle Granitoide einen kalkalkalinen Trend. Dies lässt auf eine subduktionsbezogene Schmelzbildung schliessen. Die Granitoide zeigen eine prä- syn- und postkollisionale geochemische Signatur.______Die Rb-Sr Biotitaltersdaten (Biotit – Gesamtgesteins Isochronen) liegen zwischen etwa 76 und 86 Millionen Jahren. Acht Alter liegen bei etwa 84 bis 86 Millionen Jahren. Diese werden als Abkühlalter (Schließungstemperatur bei 300±50°C) interpretiert. Die Ab- kühlung dieses Teiles der Seckau-Decke unter 300°C erfolgte daher um etwa 85 Millionen Jahre im Santon.______Mikrostrukturelle Analysen zeigen, dass der Kontaktbereich zwischen Rannach-Formation und dem darunter liegenden Basement durch einzelne Scherzonen, die Extensionsstrukturen zeigen, überprägt wird. Die Kinematik entlang dieser Scherzonen zeigt WNW- und ESE- gerichtete Kinematik. Feldspat zeigt überwiegend kataklastische Deformation, Quarz zeigt Deformationsmechanismen von Dislokationskriechen sowie dynamische Rekristallisation durch Bulging. Die Deformationsbedingungen innerhalb dieser Scher- zonen liegen daher im Bereich von etwa 300 bis 400°C. Daraus kann geschlossen werden, dass das Basement der Seckau-Decke innerhalb eines Dehnungsregimes im Zeitraum um 85 bis 80 Millionen Jahre exhumiert wurde.______

1. Introduction The understanding of the tectono-metamorphic evolution of gation of units showing an Eo-Alpine high-grade metamorphic the Austroalpine Unit has made significant progress in the last evolution including eclogite facies metamorphism, whereas few years (Schmid et al., 2004; Froitzheim et al., 2008). An the amount of structural, metamorphic, and geochronological essential key to reveal the tectonostratigraphy of Austroalpine data from other units is quite low.______nappes and the sequence of nappe emplacement was the Thöni and Jagoutz (1993), Neubauer (1994), Plasienka (1995), analysis of the Alpine and pre-Alpine metamorphic evolution and Froitzheim et al. (1996) and Neubauer et al. (2000) among of distinct nappes (e.g., Schuster et al., 2004; Handy and Ober- others, developed a picture where the Eo-Alpine metamor- hänsli, 2004). Studies on these units, however, are rather un- phism is related to the collision of the Austroalpine Unit with equally distributed. Strong emphasis was given on the investi- another continental fragment after closure of the Meliata Oce- © Österreichische Geologische Gesellschaft/Austria; download unter www.geol-ges.at/ und www.zobodat.at

Stefan PFINGSTL, Walter KURZ, Ralf SCHUSTER & Christoph HAUZENBERGER an to the south or southeast, the Austroalpine being in a lower however, is frequently taken uncritically to be related to Creta- plate position. An alternative model is given by Stüwe and ceous nappe imbrication associated with general top-to-the- Schuster (2010) where the formation of the Alpine orogenic west emplacement. This concept, elaborated about 20-30 years wedge initiated at an intracontinental subduction zone which ago, does actually not sustain critical review. Age data (e.g., Dal developed from a transform fault with a sinistral offset linking Piaz et al., 1995; Dallmeyer et al., 2008) and the sedimentary the Penninic (Alpine Tethys) Ocean and Neotethys (Meliata) record (e.g., Frisch and Gawlick, 2003) suggest a large time Ocean in Late Jurassic times.______span or time gap between the subduction of the Triassic part “Eo-Alpine” (as term for Early Cretaceous to early Late Cre- of the Neotethys Ocean (Meliata oceanic realm) at ca. 160 Ma taceous) deformation and metamorphism (e.g., Frank, 1987), (Missoni and Gawlick, 2010) and classical “Eo-Alpine” ages

Figure 1: Overview maps (A) showing the paleogeographic origin of the main tectonic units of the Alps, and (B) the tectonic subdivision of the Austroalpine unit based on Schmid et al. (2004).______© Österreichische Geologische Gesellschaft/Austria; download unter www.geol-ges.at/ und www.zobodat.at

Geochronological constraints on the exhumation of the Austroalpine Seckau Nappe (Eastern Alps) dating high pressure metamorphism around 100-90 Ma (e.g., Triassic quartzites and locally Middle Triassic greyish marbles Thöni and Jagoutz, 1992, 1993; Thöni et al., 2008). So the rauhwacke and dolomite. In the basement units the Eo-Alpine question arises whether a continuous process or distinct pha- imprint reaches sub-greenschist to epidote-amphibolite facies ses of convergence, collision, and subsequent extension for- conditions causing a retrograde overprint in most of the Vari- med the Austroalpine nappe pile as post-collisional extension scan metamorphic rocks (Neubauer et al., 1995; Schuster et was recognized to play a major role in Alpine orogeny (Ratsch- al., 2004; Hoinkes et al., 2010). Nappes of the Silvretta-Seck- bacher et al., 1989).______au Nappe System build up antiformal structures in the western In this study the lithosthratigraphic and tectonic subdivision (Schladminger Tauern) and eastern part (Seckauer Tauern) of of the Silvretta-Seckau Nappe System in the area of the Seck- the Niedere Tauern mountain ridge.______auer Tauern is reviewed. Based on new Rb-Sr biotite ages To the north the Silvretta-Seckau Nappe System is overlain the cooling history of the Seckau Complex is discussed in the by the nappes of the Greywacke zone, which consists of green- frame of regional geology.______schist facies metamorphic Paleozoic sequences, and the Ju- vavic, Tirolic-Noric and Bajuvaric Nappe Systems. The latter 2. Geological setting of the Austroalpine form the Northern Calcareous Alps, mostly comprising un-me- Unit tamorphosed to lowermost greenschist facies metamorphic The Austroalpine Unit forms a complex nappe stack of crus- Permian to Mesozoic sediments deposited on the shelf facing tal material which can be subdivided into a Lower Austroalpine originally towards the Meliata Ocean.______and Upper Austroalpine Subunit (Schmid et al., 2004; Froitz- To the south the Silvretta-Seckau Nappe System is overlain heim et al., 2008) (Fig. 1). The Lower Austroalpine Subunit by the Koralpe-Wölz Nappe System which represents an Eo- derived from the continental margin towards the Penninic (Al- Alpine metamorphic extrusion wedge (Schmid et al., 2004). pine Tethys) Ocean and was affected by extension during the Its Permian to Mesozoic cover was completely stripped off Jurassic opening and by nappe stacking during Late Creta- during an early phase of the Eo-Alpine orogenic event and it ceous to Eocene closure of this oceanic realm, respectively therefore consists exclusively of metamorphic basement nap- (Schmid et al., 2004; Froitzheim et al., 2008). It is overlying pes. The Ötztal-Bundschuh Nappe System shows a similar li- the Penninic nappes of the Eastern Alps derived from the Pen- thological composition as the Silvretta-Seckau Nappe System, ninic oceanic domain. The Upper Austroalpine Subunit repre- but is positioned on top of the Koralpe-Wölz Nappe System. sents a nappe pile which formed mainly during the Eo-Alpine Within this nappe system Eo-Alpine metamorphic grade de- event in the Early Cretaceous to early Late Cretaceous. Its creases upwards from epidote-amphibolite facies at the base lowermost part is the Silvretta-Seckau Nappe System consis- to diagenetic conditions at the top of the nappe pile. In other ting of a basement with a dominating Variscan metamorphic words, the high-grade nappes of the Koralpe-Wölz Nappe imprint and remnants of Permian to Lower Triassic cover se- System are sandwiched between nappes affected by rather quence. The basement is dominated by paragneisses (partly medium to low- grade Eo-Alpine metamorphism (for summary, migmatic) and orthogneisses with minor intercalations of mica- see Schmid et al., 2004; Froitzheim et al., 2008). The overly- schists, quartzites, amphibolites, hornblende bearing gneisses ing Drauzug-Gurktal Nappe System is made up of a Variscan and local occurrences of serpentinites and eclogites. Most of metamorphic basement, anchizonal to greenschist facies Pa- the amphibolites and orthogneisses developed from Cambri- leozoic metasedimentary sequences and by lower greenschist an to Ordovician precursors and are interpreted to reflect pre- facies or un-metamorphosed Permian to Triassic sediments Cambrian to Ordovician collision, subduction and rifting pro- (Rantitsch and Russegger, 2000).______cesses (Neubauer, 2002). Additional some Carboniferous in- trusions are present (Schermair et al., 1997). The magmatic 3. Geology of the Silvretta-Seckau Nappe inventory indicates Neoproterozoic to Ordovician educt ages System in the area of the Seckauer Tauern of the metasedimentary rocks. The medium to high grade im- The Seckau Nappe as part of the Silvretta-Seckau Nappe print in these basement rocks occurred during the Variscan tec- System (Fig. 1) covers a triangle shaped area extending in the tonometamorphic event (Neubauer et al., 1999). In the struc- north from the Bösenstein massif and Seckauer Tauern in the turally deepest part of the westernmost element represented eastern part of the Niedere Tauern to the Fischbacher Alps by the Silvretta Nappe also a Permian metamorphic overprint over nearly 100 km. There it is cut off by the sinistral, east- can be recognized (Schuster et al., 2004). The post-Variscan west trending Trofaiach fault. Its continuation is represented cover is best preserved in the Silvretta Nappe which is over- by the Troiseck Floning Nappe (Neubauer, 1988). The southern lain by un-metamorphosed Permian to Triassic sediments in edge is at Ameringkogel in the northern part of the Koralpe the Landwasser and Ducan synclines and a more or less stra- mountain ridge (Becker, 1979, 1980). The north south-exten- tigraphic contact to the Permomesozoic sequences of the Ba- sion is about 50 km.______juvaric Nappe System (Nowotny et al., 1993; Nagel, 2006). The nomenclature used in literature for the crustal piece re- Further to the east tectonically truncated sequences show an ferred to as Seckau Nappe in here is quite complicated and Eo-Alpine greenschist facies overprint. They comprise only Per- confusing. In the past it is attributed to the so called Muriden mian metaconglomerates and metapelites as well as Lower unit in contrast to the overlying Koriden unit. However, this © Österreichische Geologische Gesellschaft/Austria; download unter www.geol-ges.at/ und www.zobodat.at

Stefan PFINGSTL, Walter KURZ, Ralf SCHUSTER & Christoph HAUZENBERGER subdivision was never extended over the whole Austroalpine For the eastern continuation „Mugel-Rennfeldzug“ (Schwin- Unit (e.g., Neubauer and Frisch, 1993). Terms used for the ner, 1951, S. 110; Metz, 1971, S. 56; Tollmann, 1977, S. 222) area of the Bösenstein massiv and the Seckauer Tauern, which and „Rennfeld Mugel Kristallin“ (Neubauer, 1988) can be found. is dominated by orthogneisses, paragneisses and migmatic In the southern part two units had been distinguished. A lower paragneisses (Metz, 1976) were “Seckauer Gneis“ (Schmidt, Amering-Komplex (Flügel and Neubauer, 1984) („Ammering- 1921, p. 103 ff), „Seckauer Masse“, „Seckauer Massiv“ (Toll- serie bzw. Gneiskomplex“ in Becker, 1980) formed by biotite- mann, 1977), „Kristallin der Seckauer Tauern“ (Metz, 1980) plagioclase paragneisses and some orthogneis intercalations and „Seckauer Kristallin“ (Scharbert, 1981, Tollmann, 1977, p. is overlain by the Speik-Komplex (Flügel and Neubauer, 1984), 287). The term Seckau Complex (Faryad and Hoinkes, 2001, („Speikserie bzw. Amphibolitkomplex“ in Becker, 1980) domi- Gaidies et al., 2006) should be favoured as it matches recent nated by amphibolites with some ultramafic rocks and an eclo- lithostratigaphic standards.______gite at Hochgrössen mountain (Fig. 1). However, the individual

Figure 2: (A) Geological map of the study area in the Seckauer Tauern, based on Metz (1967, 1979), Flügel and Neubauer (1984) and Schubert et al. (2009). (B) Enlargement of the sampling area with sites of samples and the revealed Rb-Sr isochron ages.______© Österreichische Geologische Gesellschaft/Austria; download unter www.geol-ges.at/ und www.zobodat.at

Geochronological constraints on the exhumation of the Austroalpine Seckau Nappe (Eastern Alps) complexes are not well defined and their boundaries are not because there is a Rannach-Group defined in the nappes of clear. The overlying Permian to Lower Triassic metasediments the Graz Paleozoic just 30 km to the southeast (Flügel, 2000). are summarized as Rannach-Formation (Flügel and Neubauer, In the northern part of the Seckau Complex the Rannach-For- 1984),(after “Rannachserie” from Schwinner, 1929). It has to mation is dipping northward, whereas in the south the Speik be mentioned that the term Rannach-Formation is problematic Complex is the uppermost element. Further, the Gaal Zone (Metz, 1971, “Gaaler Schuppenzo- ne”) mainly composed of the Speik Complex and overlying Permian to Lower Triassic metasediments is tectonically overlying the northwes- ternmost part of the Seckau Nappe in the Bösenstein area.______As mentioned before the pre-Al- pine basement of the Seckau-Com- plex is formed by orthogneisses, paragneisses and migmatic paragneisses. The partly porphyric orthogneisses derived from tonalites, granodi- orites as well as granites. In general these units form a coherent suite of I-type granitoids (Schermaier et al., 1997; Pfingstl, 2013). A-type, and transitional S- and I- type granitoids are represented to minor extent. Ba- sed on Rb-Sr whole rock (~ 350 Ma) and white mica ages (~330 Ma) and the low grade Eo-Alpine metomor- phic overprint of the Permomesozo- ic metasediments above, the orthogneisses are thought to have formed during the Variscan tectonometa- morphic event (Scharbert, 1981). An additional Rb-Sr whole rock age of 432 +/- 16 Ma argues for a pre-Va- riscan, possibly Ordovician intrusion age. To clarify the ages of the individual intrusive bodies, however, mo- dern zircon ages are necessary.___ The metamorphic conditions in the Seckau Nappe show a slight increase towards the south. In the northwest the transgressive Permian to Lower Triassic metasediments (Rannach- Formation) are characterised by the assemblage muscovite + chloritioid + chlorite + quartz, indicating upper greenschist facies conditions (520°C at 0.9 Gpa) (Faryad and Hoinkes, 2001). In the Gleinalm area in the south lower amphibolite facies conditions are expected from amphibolite assemblages including garnet + Figure 3: Harker diagrams with selected major and trace elements. The porhphyric samples amphibole + plagioclase and interca- (black squares) have low SiO2 values but are high in CaO, P2O5, Sr and Ce. The S-type orthogneiss lated micaschists with garnet, stau- samples SP62 and SP79 (gray triangles with black outline) are relatively enriched in SiO2, K2O and

Rb, while low in CaO, Na2O, P2O5, Sr and Ce.______rolite and kyanite. For the eastern © Österreichische Geologische Gesellschaft/Austria; download unter www.geol-ges.at/ und www.zobodat.at

Stefan PFINGSTL, Walter KURZ, Ralf SCHUSTER & Christoph HAUZENBERGER part no data are available, but as in the eastern continuation of the unit, represented by the Troiseck Floning Nappe, pre- Alpine Ar-Ar muscovite ages are preserved, decreasing conditions can be expected (Dallmeyer et al., 1998).______Timing of the peak of Eo-Alpine metamorphism in the Seck- au Nappe is not constrained with age data and also for the cooling through greenschist facies conditions the data are scarce. Three Rb-Sr biotite ages are in the range of 70 to 77 Ma (Scharbert, 1981). Unexpected high fission track ages were described by Hejl (1997). Orthogneiss samples from the summit and valley areas of the Seckau Tauern yielded apatite fission track ages of ca. 60 Ma and 44 Ma, respectively. These fission track data indicate that after fast cooling during Late Cretaceous times the Seckau Nappe stayed at nearly con- stant temperature in the order of 80°C until the late Oligocene, followed by a phase of faster cooling, and a phase of slow cooling during Miocene times. The fission track data exclude Figure 4: Total alkali (Na2O + K2O) against SiO2 classification any post-Cretaceous metamorphic overprint within the Seckau diagram after Middlemost (1994). Square and star symbols indicate samples from the “Untere Bodenhuette”, samples with triangular sym- Nappe. During the whole Cenozoic the highest portions of the bols are from the “Ingering Gaal” area.______Seckau Nappe therefore resided at a depth of less than 3000 meters at temperatures much cooler than the conditions of anchimetamorphism (Hejl, 1997).______ISOPLOT/Ex (Ludwig, 2001, 2003) assuming an error of 1% on the 87Rb/86Sr ratio.______4. Analytical techniques The chemical composition of whole rock samples was de- Mechanical and chemical sample preparation for Rb and Sr termined by XRF. The finely powdered samples were dried at isotope analyses were performed at the Geological Survey of least 2 hours at 105 °C prior to further treatment. About 1g of Austria in Vienna. Minerals were separated by standard me- sample was used for LOI determination at 1030 °C. For deter- thods of crushing, grinding, sieving and magnetic separations. mination of major and selected trace elements a fused bead Weights of samples used for dissolution were about 100 mg was prepared by using 1g of dried powder and 7g of dilithi- for whole rock powder and about 200 mg for biotite. Chemical umtetraborate. The glass bead was fused at ~1300°C with a sample preparation follows the procedure described by Sölva semi-automatic VAA-2 fusion machine from HD-Electronic. et al. (2005). Isotopic measurements were done at the Depart- About 60 international reference materials were used to cali- ment of Lithospheric Research at the University of Vienna. Rb brate the Bruker Pioneer S4 XRF, located at the Institute of and Sr concentrations were determined by isotope dilution us- Earth Sciences, Karl-Franzens-University Graz. The USGS ing mixed Rb-Sr spikes. Rb ratios were measured with a Fin- reference material GSP-2 is analysed routinely as monitor nigan® MAT 262 MC-TIMS, whereas Sr ratios were analysed standard which can be reproduced within the given errors.__ with a ThermoFinnigan® Triton MC-TIMS. Sr was run from Re double filaments, whereas Rb was evaporated from a Ta fila- 5. Analytical data ment. Total procedural blanks are < 1ng for Rb and Sr. During The mainly foliated orthogneisses are from two localities, na- the periods of measurements the NBS987 standard yielded mely in the Untere Bodenhütte (square and star symbols in Fig. 86Sr/87Sr = 0.710248±4 (n=17; 2σ standard deviation) on the 3) and Ingering Gaal area (triangles in Fig. 4). The geographic Triton TI. Isochron ages were calculated with the software location of the samples and their lithological / geochemical

Table 1: Sites of samples analysed for Rb-Sr geochronology on biotite. © Österreichische Geologische Gesellschaft/Austria; download unter www.geol-ges.at/ und www.zobodat.at

Geochronological constraints on the exhumation of the Austroalpine Seckau Nappe (Eastern Alps)

classification are given in Table 1, analytical data in Table 3.______Samples from the Untere Boden- hütte area include two porhphyric and only weakly foliated samples (SP58, SP59). Compared to the other samples described below, SP58 and

SP59 show lower SiO2, Na2O and Rb concentrations but show the highest

CaO, P2O5, Sr, and Ce contents. One sample (SP78), which did not con- tain biotite and thus was not dated, contains muscovite and geochemical data are plotted for comparison in Figure 4. This sample shows slightly different chemical features and con-

tains the highest SiO2 values of all samples with 74.8 wt.%. From the Ingering Gaal area four samples were taken. Two samples display

higher SiO2, K2O and Rb while CaO,

P2O5, Sr, and Ce values are very low. Based on geochemical and Sr isotope data both sample were identified to belong to a granitic suite with S-type affinity. However, most analyzed samples can be geoche- mically classified as I-type granites with respect to Chappell and White (1974, 2001) (Pfingstl, 2013) (Table 1). Sample SP60 from the Feistritz valley has been identified as metamorphosed mafic dike. Following the classification by Middlemost (1994) by application of the TAS diagram for plutonic rocks samples SP53, SP62, SP79, and SP78 can be classified as granites, and SP54, SP56, SP58, SP80 and SP81 as granodi- orites (Figure 4). Only sample SP59 falls into the quartz monzodiorite field. The metamorphosed mafic dike SP 60 has a basaltic trachyandesite/ quartz-monzodiorite composition.__ The results of geochronological analysis are summarized in Table 2, related Rb-Sr biotite ages and isochrons are displayed in Figure 5. The investigated biotites show brown to yellowish-brown pleochroism. Some separated from orthogneisses show typical rutile exsolutions (“Sagenitgit- ter”), indicating higher Ti-contents of the primary magmatic biotites. With- Figure 5: Selected Rb-Sr biotite – whole rock ages from orthogneisses of the Seckau Complex. in the eastern part of the investiga- © Österreichische Geologische Gesellschaft/Austria; download unter www.geol-ges.at/ und www.zobodat.at

Stefan PFINGSTL, Walter KURZ, Ralf SCHUSTER & Christoph HAUZENBERGER ted area near to Bodenhütte ages range from 83.3±0.8 Ma to ged grain boundaries. Aggregates of fine-grained dynamically 84.8±0.9 Ma (SP53, SP54, SP56, SP58, SP59, SP60). Taking recrystallized grains (grain size ~ 0.05 - 0.1 mm) developed into account the standard deviations (Table 2), these ages are along shear bands due to secondary grain size reduction; bul- more or less identical within error. A similar age of 83.2±0.8 ging is assumed to be the main deformation mechanism.____ Ma (SP62) has also been observed from the southern part of Along the contact between the Seckau Complex and the the investigated area. An age from the Hochreichart (Fig. 2) Rannach-Formation both K-feldspar and plagioclase show yielding 79.1±0.8 Ma (SP79) is significantly younger. In the high undulatory extinction, and, within domains along shear western part of the area of investigation, west of the Ingering bands, curved twins. The feldspars are characterized by cata- valley, two different ages were observed. An age of 82.7±0.8 clastic deformation mechanisms, documented by the formation Ma (SP81) is widely identical to the bulk of ages yielded in of extensional fractures and shear fractures (Fig. 6c). Exten- the eastern part, whereas an age of 74.9±0.7 Ma (SP80) is sional fractures are filled with quartz and calcite. Fragments significantly younger.______confined by shear fractures were rotated towards the direction Calculation of a regression line including all whole rocks ana- of shear that is accommodated by single and/or conjugate lyses of granitic and granodioritic rocks results in an age value sets of shear bands. Along these shear bands (Fig. 6b) quartz of 507±60 Ma with an initial isotopic ratio of 0.7045±0.0016._ is characterized by grain size reduction due to dynamic recrystallization. Biotite is aligned along these shear bands; within 6. Microstructures distinct domains biotite recrystallized and was newly formed. Within the deeper levels of the Seckau Complex quartz is Additionally biotite is transformed to chlorite. In case of the characterized by partly annealed fabrics. Equigranular grains occurrence of single sets the shear bands indicate a top-to- generally show a polygonal shape (Fig. 6a). Quartz grains show the WNW sense of shear. Antithetic sets with top-to-the ESE grain sizes of 0.2 - 0.4 mm, the average grain size equals about kinematics can be observed as well.______0.3 mm. Irregular and lobate grain boundaries are weakly de- As feldspar is deformed by cataclastic deformation mecha- veloped; generally the grain boundaries are straight or slightly nisms, whereas quartz shows deformation mechanisms of curved and form 120°- triple junctions. In places, the grains dislocation creep and bulging dynamic recrystallization, we were subsequently affected by low-temperature deformation, assume that the conditions of deformation along the higher and, therefore, show undulatory extinction.______part of the Seckau Complex and within the Rannach-Forma- Along the contact to the Rannach-Formation quartz grains tion are in the range of 300°C to 400°C.______are slightly elongated (Fig. 6b). Ribbon grains formed along shear bands. The quartz grains are characterized by undula- 7. Discussion tory extinction, the formation of subgrains, deformation lamel- The Rb-Sr biotite age data presented in this study range lae and deformation bands, and by sutured, lobate, and bul- from 74.9±0.8 to 84±0.9 Ma, with eight samples showing ages

Table 2: Rb-Sr isotopic data on biotite and whole rock from the Seckau Nappe. Analytical techniques are described in text. Isochron ages were calculated from biotite and corresponding whole rock assuming an error of ±1% on the 87Rb/86Sr ratio.______© Österreichische Geologische Gesellschaft/Austria; download unter www.geol-ges.at/ und www.zobodat.at

Geochronological constraints on the exhumation of the Austroalpine Seckau Nappe (Eastern Alps)

around 82 to 84 Ma (Fig. 5) (Table 2). A sample from Hochreichart mountain has an age of 79.1±0.8 Ma, another sample (SP80) west of the In- gering valley yielded only 76.3±0.8 Ma. The latter seems to be excepti- onal as it is sampled only about 250 meters from sample SP81, which has an age of 82.7±0.8 Ma and as both samples show a nearly identical mi- neralogical and chemical composition. It is obvious that both samples underwent the same cooling history and we attribute the difference in age to a slight contamination by chlorite, grain size and/or weathering ef- fects. According to Jäger (1979) Rb- Sr biotite-ages are interpreted to date cooling below 300±50°C. Therefore the investigated part of the Seckau Complex cooled down below ~300°C at about 85 Ma in the Santonian.__ A regression line calculated from all whole rock analyses of granites, granodiortites and the quartz monzodiorite yielded an age value of 507±60 Ma. As obtained from the geochemical data the rocks show both I-type and S-type affinities, and therefore most probably not all of the investigated rocks belong to a co-ge- netic magmatic suite. However, ex- cept the granite SP53 all data points fit well to a regression line defining an age value of 517±16 Ma with an initial 87Sr/86Sr ratio of 0.7047±6. I-

Figure 6: (a) Microstructure (cros- sed polarizing filters) of a granitic orthogneiss from the Seckau basement, not affected by extensional shear. Quartz is characterized by equigranular grain shape, nearly straight grain boundaries and only subordinate undulatory extinction. (b) Microstructure characteristic for the contact domain between the Seck- au basement and the Rannach-Forma- tion. Shear bands indicate top-to-the west sense of shear. Quartz along shear bands is characterized by highly reduced grain size. Arrows indicate the sense of shear. (c) Microstructure of a granitic orthogneiss from the Seckau basement, immediately beneath the contact to the Rannach-Formation. Quartz is characterized by undulatory extinction and formation of multiple subgrains, Feldspar is characterized by undulatory extinction, bent twins, and fracturing.______© Österreichische Geologische Gesellschaft/Austria; download unter www.geol-ges.at/ und www.zobodat.at

Stefan PFINGSTL, Walter KURZ, Ralf SCHUSTER & Christoph HAUZENBERGER

Type and S-Type rocks are aligned on the regression line with- caused cooling below 300±50°C at 85 Ma in the Late Creta- out any special systematics and therefore it might represent a ceous. This coincides with the formation of the Gosau sedi- geological meaningful errorchrone indicating crystallization of mentary basins in the central Eastern Alps (Neubauer et al., these rocks during Early Paleozoic times. Based on Rb-Sr da- 1995; Dallmeyer et al., 1998; Faupl and Wagreich, 2000).___ ta also Scharbert (1981) and Schermaier et al. (1997) argued The Rb-Sr biotite ages by Scharbert (1981), which are slight- for an early Paleozoic age of some igneous rock associations ly younger, were measured on samples taken from the more in the Seckau Complex.______southern part of the Seckauer Tauern. They may indicate a Referring to microstructural observations, the contact be- trend to later cooling towards the south.______tween the Seckau Complex and the Rannach-Formation was Geochronological data from the Seckau Complex collected strongly overprinted by the formation of distinct shear zones from literature give additional information. An 40Ar/39Ar amphi- that are characterised by extensional fabrics. Kinematics is bole age measured on an eclogite amphibolite of the Speik characterized by conjugate sets of top-to-the-WNW and top- Complex of the Gaal Zone in the Bösenstein massif yielded to-the-ESE. about 390 Ma (Faryad et al., 2002). This pre-Alpine age had A comprehensive view of the ages and structural observa- been preserved as metamorphic conditions determined for tions presented in this study shows that deformation tempera- the northern part of the Silvretta-Seckau Nappe System are tures overlap with the range of Rb-Sr biotite closure tempera- around 500°C and are therefore in the range of the closure ture. Therefore we assume that top-to-the WNW extensional temperature of the K-Ar isotopic system in amphibole (von shearing is related to exhumation of the Seckau Nappe and Blanckenburg and Villa, 1988; von Blanckenburg et al., 1989).

Table 3: Chemical composition of selected rocks from Untere Bodenhütte area (SP53-SP78), Ingering Gaal (SP62-SP81), and a mafic dike from Feistritz valley. © Österreichische Geologische Gesellschaft/Austria; download unter www.geol-ges.at/ und www.zobodat.at

Geochronological constraints on the exhumation of the Austroalpine Seckau Nappe (Eastern Alps)

Also Rb-Sr muscovite ages from the area around Bodenhütte continuation of the Seckau Nappe System as well.______yielded Variscan ages of about 330 Ma Scharbert (1981). With Fission track data from the Gleinalm mountain range measu- respect to the closure temperature of the Rb-Sr isotopic sys- red on sphene, zircon and apatite yielded ages of ca. 69 Ma, tem in muscovite (Jäger, 1979) Alpine peak metamorphic con- 65 Ma, and 34 Ma, respectively (Neubauer et al., 1995; Dunkl ditions of less than, or about 500 °C are indicated. In contrast et al., 2005). The fission track ages therefore support the trend an 40Ar/39Ar age from the Speik Complex in the Gleinalm area with earlier cooling in the Seckauer Tauern with respect to the yielded 95.4±1.2 Ma (Neubauer et al., 1995). This age reflects Gleinalm mountain range: The zircon fission track age of 65 total resetting of the K-Ar isotopic system in hornblende du- Ma representing cooling below the partial annealing zone (< ring eo-Alpine epidote-amphibolite facies metamorphism at 240°C) (e.g., Wölfler et al., 2010) is just slightly older than the 550-600°C at 0.9-1.0 GPa (Faryad and Hoinkes, 2003)._____ apatite fission track ages from the Seckauer Tauern yielding In the Gleinalm mountain range the Speik Complex is over- 60-44 Ma (Hejl, 1997). The latter indicates that the northern lain by the Wölz and Rappold Complexes being parts of the part of the Seckau Complex cooled below the apatite fission Koralpe-Wölz Nappe System. 40Ar/39Ar muscovite ages from track partial retention zone (120° - 60°C; Green et al., 1986; there are in the range of 84 to 87 Ma (Neubauer et al., 1995). Wolf et al., 1996) in Oligocene times. The apatite fission track These muscovite ages are in the range of the new Rb-Sr coo- age from the Gleinalm is 34 Ma (Dunkl et al., 2005) and indi- ling ages presented in this study. In the eastern part of the Eas- cates cooling 25-30 Ma later than in the north.______tern Alps, 40Ar/39Ar muscovite ages from the Silvretta-Seckau Compared to the cooling path of the basement rocks in the Nappe System (the so-called Troiseck-Floning Zug) indicate Seckau Tauern described above, cooling of the adjacent Glein- cooling of this unit between ca. 88 and 82 Ma (Dallmeyer et alm mountain range southeast of the Seckau Tauern appears al., 1998). Despite the fact that the closure temperature of the to be more continuous (Fig. 7). The data set from the Gleinalm Ar-isotopic system in white mica has been reported to be with- mountain range, however, is also more complete than those in a rather wider temperature range from ca. 350°C (Lips et from the Seckauer Tauern. The question arises whether the al., 1998) to 425 ± 25 °C (Harrison et al., 2009), ca. 450 °C Seckau and the Gleinalm domains of the Silvretta-Seckau Nap- (Hames and Bowring, 1994; Kirschner et al., 1996) and ca. pe System represent a continuous nappe sheet or two inde- 500°C (Hammerschmidt and Frank, 1991; Hames and Cheney, pendent parts with a prominent shear zone in between which 1997) and depends on grain-size, chemical composition and decoupled them during their exhumation. Based on mapping cooling rate (Villa, 1998; Warren et al., 2012), it is higher than no major fault is known yet which could have caused the de- those for Rb-Sr in biotite (300±50°C) (Jäger, 1979). This indi- coupling, until the sinistral Mur-Mürz fault developed during la- cates earlier cooling of the Seckau Nappe below 350°C with- teral extrusion of the Eastern Alps (Ratschbacher et al., 1991) in the Seckauer Tauern than in the Gleinalm mountain range. during Miocene times. If this is the case, the Seckau Complex Moreover, this evolution can be reconstructed for the eastward was a south dipping element from Cretaceous until Paleogene time when extensional tectonics and/or pronounced erosion in the south caused a tilting in a more horizontal position._____

8. Concluding remarks According to the new Rb-Sr biotite cooling ages presented in this study, the Seckau Complex cooled below~300°C at about 85 Ma in the Santonian. The contact of the Seckau Complex to the overlying Permotriassic metasediments of the Rannach- Formation is characterized by lower greenschist facies shear zones with top-to-the WNW displacement. As these shear zones developed at temperatures in the range of the Rb-Sr isotopic system in biotite we assume that cooling of the northern part of the Seckau Nappe is due to extensional shearing within the higher structural levels of the Seckau Nappe. Cooling ages indicate earlier cooling in the northern than in the southern part of the Seckau Nappe.______

Figure 7: T-t paths for the different parts of the Seckau Nappe: Acknowledgements Prior to the Eo-Alpine metamorphic peak the Seckau Complex and the We greatfully acknowledge Stanislaw Grabala (Geological overlying Permotriassic sediments were heated. The Eo-Alpine metamorphic peak was reached at 90-100 Ma. Rocks from the Seckauer Survey of Austria) for help with the biotite separation. Monika Tauern experienced lower peak conditions as those in the Gleinalm Horschinegg (University of Vienna) is thanked for her help in area (about 500 °C) and cooled earlier than the latter. Diagram based measuring the Sr- isotopic ratios. Werner Beke (University of on data from Dunkl et al. (2005), Faryad and Hoinkes (2001, 2003), Hejl (1997), Neubauer et al. (1995) and Scharbert (1981). Greyish Graz) is thanked for his support during sample preparation for lines inticate expected parts of T-t path. For explanation see text.___ the geochemical analyses.______© Österreichische Geologische Gesellschaft/Austria; download unter www.geol-ges.at/ und www.zobodat.at

Stefan PFINGSTL, Walter KURZ, Ralf SCHUSTER & Christoph HAUZENBERGER

References Flügel, H. W., 2000. Die lithostratigraphische Gliederung des Becker, L.P., 1979. Geologische Karte der Republik Österreich Paläozoikums von Graz (Österreich). Österreichische Akade- 1:50.000, Blatt 162 Köflach. Geologische Bundesanstalt, Wien. mie der Wissenschaften / Schriftenreihe der Erdwissenschaft- lichen Kommissionen, 13, 7-59. Becker, L.P., 1980. Erläuterungen zu Blatt 162 Köflach. Geo- logische Karte der Republik 1:50.000, Geologische Bundes- Flügel, H. W. and Neubauer, F. R., 1984. Geologische Karte anstalt, Wien. der Steiermark 1:200.000. Geologische Bundesanstalt, Wien.

Chappell, B. W. and White, A. J. R., 1974. Two contrasting Frank, W., 1987. Evolution of the Austroalpine elements in the granite types. Pacific Geology, 8, 173-174.______Cretaceous. In: H. W. Flügel and P. Faupl (eds.), Geodynamics of the Eastern Alps, 379-406. Vienna, Deuticke.______Chappell, B. W. and White, A. J. R., 2001. Two contrasting granite types: 25 years later. Australian Journal of Earth Sci- Frisch, W. and Gawlick, H.-J., 2003. The nappe structure of ences, 48, 489-499. http://dx.doi.org/10.1046/j.1440-0952. the central Northern Calcareous Alps and its disintegration du- 2001.00882.x ring Miocene tectonic extrusion - a contribution to understanding the orogenic evolution of the Eastern Alps. International Dal Piaz, G. V., Martin, S., Villa, I. M., Gosso, G. and Marschal- Journal of Earth Sciences, 92, 712-727.______ko, R., 1995. Late Jurassic blueschist facies pebbles from the Western Carpathian orogenic wedge and paleostructural impli- Froitzheim, N., Plasienka, D. and Schuster, R., 2008. Alpine cations for western Tethys evolution. Tectonics, 14, 874-885._ tectonics of the Alps and Western Carpathians. In: T. McCann (ed.) The Geology of Central Europe, Volume 2: Mesozoic and Dallmeyer, R.D., Handler, R., Neubauer, F. and Fritz, H., 1998. Cenozoic. 1141-1232. Geological Society of London.______Sequence of thrusting within a thick-skinned tectonic wedge: evidence from 40Ar/39Ar and Rb-Sr ages from the Austroal- Froitzheim, N., Schmid, S. M. and Frey, M., 1996. Mesozoic pine nappe complex of the Eastern Alps. Journal of Geology, paleogeography and the timing of eclogite facies metamor- 106, 71-86. phism in the Alps: A working hypothesis. Eclogae geologicae Helvetiae, 89, 81-110. Dallmeyer, R. D., Neubauer, F. and Fritz, H., 2008. The Meli- ata suture in the Carpathians: regional significance and impli- Green, P. F., Duddy, I. R., Gleadow, A. J. W., Tingate, P. R. cations for the evolution of high-pressure wedges within colli- and Laslett, G. M., 1986. Thermal annealing of fission tracks sional orogens. Geological Societyof London Special Publica- in apatite 1. A qualitative description. Chemical Geology, 59, tions, 298, 101-115. http://dx.doi.org/10.1144/SP298.6______237-253.

Dunkl, I., Kuhlemann, J., Reinecker, J. and Frisch, W., 2005. Hames, W. E. and Browning, S. A., 1994. An empirical evalu- Cenozoic relief evolution of the Eastern Alps - Contraints from ation of the argon diffusion geometry in muscovite. Earth and apatite fission track age-provenance of Neogene intramontane Planetary Science Letters, 124, 161-167.______sediments. Austrian Journal of Earth Sciences, 98, 92-105.__ Hames, W.E. and Cheney, J.T., 1997. On the loss of 40Ar* from Faupl, P. and Wagreich, M., 2000. Late Jurassic to Eocene muscovite during polymetamorphism. Geochimica et Cosmo- Palaeogeography and Geodynamic Evolution of the Eastern chimica Acta, 61, 3863-3872. Alps. Mitteilungen der Österreichischen Geologischen Gesell- Hammerschmidt, K. and Frank, E., 1991. Relics of high pres- schaft, 92, 79-94. sure metamorphism in the Lepontine Alps (Switzerland) - 40Ar/ Faryad, S. W. and Hoinkes, G., 2001. Alpine Chloritoid and 39Ar and microprobe analyses on K-micas. Schweizerische mi- Garnet from the Hochgrössen Massif (Speik Complex, Eas- neralogische und petrographische Mitteilungen, 71, 261-274. tern Alps). Mitteilungen der Österreichischen Geologischen Handy, M. R. and Oberhänsli, R., 2004. Explanatory notes to Gesellschaft, 146, 387-396. the map: metamorphic structure of the Alps: age map of the Faryad, S. W. and Hoinkes, G., 2003. P-T gradient of Eo-Al- metamorphic structure of the Alps - tectonic interpretation and pine metamorphism within the Austroalpine basement units outstanding problems. Mitteilungen der Österreichischen Mine- east of the Tauern Window (Austria). Mineralogy and Petrology, ralogischen Gesellschaft, 149, 201-225.______77, 129-159. http://dx.doi.org/10.1007/s00710-002-0196-1___ Harrison, T. M., Célélier, J., Aikman, A. B., Hermann, J. and Faryad, S.W., Melcher, F.; Hoinkes, G., Puhl, J., Meisel, T. and Heizler, M.T., 2009. Diffusion of 40Ar in muscovite. Geochimica Frank, W., 2002. Relics of eclogite facies metamorphism in the et Cosmochimica Acta, 73, 1039-1051. http://dx.doi.org/10.1016 Austroalpine basement, Hochgrössen (Speik complex), Austria. /j.gca.2008.09.038 Mineralogy and Petrology, 74, 49-73. http://dx.doi.org/10.1007/ Hejl, E., 1997. 'Cold spots' during the Cenozoic evolution of s710-002-8215-9 the Eastern Alps: thermochronological interpretation of apatite fission-track data. Tectonophysics, 272, 159-173.______© Österreichische Geologische Gesellschaft/Austria; download unter www.geol-ges.at/ und www.zobodat.at

Geochronological constraints on the exhumation of the Austroalpine Seckau Nappe (Eastern Alps)

Hoinkes, G., Koller, F., Demeny, A., Miller, CH., Schuster, R., Nagel, T., 2006. Structure of Austroalpine and Penninic units in Thöni, M., Kurz, W., Krenn, K. and Walter, F., 2010. Metamor- the Tilisuna area (Eastern Rhätikon, Austria): Implications for phism in the Eastern Alps. Acta Mineralogica-Petrographica, the paleogeographic position of the Allgäu and Lechtal nappes. Field Guide Series, 1, 1-47.______Eclogae Geologicae Helvetiae, 99, 223-235. http://dx.doi.org/ 10.1007/s00015-006-1192-y Jäger, E., 1979. Introduction to Geochronology. In: E. Jäger and J.C. Hunziger (eds). Lectures in Isotope Geology, 1-12, Neubauer, F., 1988. Bau und Entwicklungsgeschichte des Springer, Berlin Heidelberg New York.______Rennfeld-Mugel- und des Gleinalm-Kristallins (Ostalpen). Ab- handlungen der Geologischen Bundesanstalt, 42, 1-137.____ Kirschner, D. L., Cosca, M. A., Masson, H. and Hunziker, J. C., 1996. Staircase 40Ar/39Ar of fine-grained white mica: Ti- Neubauer, F., 1994. Kontinentkollision in den Ostalpen. Geo- ming and duration of deformation and empirical constraints on wissenschaften, 12, 136-140.______Argon diffusion. Geology, 24, 747-750.______Neubauer, F., 2002. Evolution of late Neoproterozoic to early Ludwig, K.R., 2001. Isoplot/Ex version 2.49. A Geochronologi- Paleozoic tectonic elements in Central and Southeast Europe- cal Toolkit for Microsoft Excel. Berkeley Geochronology Cen- an Alpine mountain belts: review and synthesis. Tectonophy- ter Special Publication N0. 1a.______sics, 352, 87-103. http://dx.doi.org/10.1016/S0040-1951(02) 00190-7 Ludwig, K.R., 2003. Isoplot/Ex version 3.0. A geochronological toolkit for Microsoft Excel. Berkeley Geochronological Centre Neubauer, F., Dallmeyer, R. D., Dunkl, I. and Schirnik, D., 1995. Special Publication, Berkeley, 70 p.______Late Cretaceous exhumation of the metamorphic Gleinalm dome, Eastern Alps: kinematics, cooling history and sedimen- Lips, A. L. W., White, S. H. and Wijbrans, J. R., 1998. 40Ar/39Ar tary response in a sinistral wrench corridor. Tectonophysics, laserprobe direct dating of discrete deformational events: a con- 242, 79-98. tinous record of early Alpine tectonics in the Pelagonian Zone, NW Aegean area, Greece. Tectonophysics, 298, 133-153.___ Neubauer, F. and Frisch, W., 1993. The Austroalpine metamorphic basement east of the Tauern Window. In: J. von Raumer Metz, K., 1967. Geologische Karte der Republik Österreich and F. Neubauer (eds.), Pre-Mesozoic Geology in the Alps, pp. 1:50.000, Blatt 130-131 Oberzeiring-Kalwang. Geologische 515-536. Springer (Berlin). Bundesanstalt, Wien. Neubauer, F., Genser, J. and Handler, R., 2000. The Eastern Metz, K., 1971. Das Problem der Grenzzone zwischen den Alps: Result of a two-stage collision process. Mitteilungen der Wölzer Glimmerschiefern und Ennstaler Phylliten. Carinthia II, Österreichischen Geologischen Gesellschaft, 92, 117-134.__ Sonderheft 28, 159-166. Neubauer, F., Hoinkes, G., Sassi, F.P., Handler, R., Höck, V., Metz, K., 1976. Der geologische Bau der Seckauer und Rot- Koller, F. and Frank, W., 1999. Pre-Alpine metamorphism in tenmanner Tauern. Jahrbuch der Geologischen Bundesanstalt, the Eastern Alps. Schweizerische mineralalogische und petro- 119, 151-205. graphische Mitteilungen, 79, 41-62.______Metz, K., 1976. Der geologische Bau der Wölzer Tauern. Mit- Nowotny, A., Pestal, G. and Rockenschaub, M.J., 1993. Die teilungen des Naturwissenschaftlichen Vereins für Steiermark, Landecker Quarzphyllit- und Phyllitgneiszone als schwächer 106, 51-75. metamorpher Anteil des Silvrettakristallins. Jahrbuch der Geo- Metz, K., 1979. Geologische Karte der Republik Österreich logischen Bundesanstalt, 135, 867-872.______1:50.000, Blatt 129 Donnersbach. Geologische Bundesan- Pfingstl, S., 2013. Tektonische und metamorphe Entwicklung stalt, Wien. des Seckauer Kristallins.- Masterarbeit, Naturwisenschaftliche Metz, K., 1980. Erläuterungen zu Blatt 129 Donnersbach. Geo- Fakultät, Universität Graz, 121 pp.______logische Bundesanstalt Wien, 35 pp.______Plasienka, D., 1995. Passive and active margin history of the Middlemost, E. A. K., 1994. Naming materials in magma/igne- northern Tatricum (Western Carpathians, Slovakia). Geologi- ous rock system. Earth Science Reviews, 37, 215-224._____ sche Rundschau, 84, 746-760.______

Missoni, S. and Gawlick, H.-J., 2010. Evidence for Jurassic Rantitsch, G. and Russegger, B., 2000. Thrust-Related Very subduction from the Northern Calcareous Alps (Berchtesga- Low Grade Metamorphism Within the Gurktal Nappe Complex den; Austroalpine, Germany). International Journal of Earth (Eastern Alps). Jahrbuch der Geologischen Bundesanstalt, 142, Sciences, http://dx.doi.org/10.1007/s00531-010-0552-z. ____ 219-225.

Ratschbacher, L., Frisch, W., Neubauer, F., Schmid, S. M. and Neugebauer, J., 1989. Extension in compressional orogenic belts. The eastern Alps. Geology, 17, 404-407.______© Österreichische Geologische Gesellschaft/Austria; download unter www.geol-ges.at/ und www.zobodat.at

Stefan PFINGSTL, Walter KURZ, Ralf SCHUSTER & Christoph HAUZENBERGER

Ratschbacher, L., Frisch, W., Linzer, H.-G. and Merle, O., 1991. Thöni, M., Miller, C., Blichert-Toft, J., Whitehouse, M. J., Kon- Lateral Extrusion in the Eastern Alps. Part 2: Structural Analy- zett, J. and Zanetti, A., 2008. Timing of high-pressure meta- sis. Tectonics, 10, 257-271.______morphism and exhumation of the eclogite type-locality (Kup- plerbrunn-Prickler Halt, Saualpe, south-eastern Austria): con- Scharbert, S., 1981. Untersuchungen zum Alter des Seckauer straints from correlations of the Sm-Nd, Lu-Hf, U-Pb and Rb- Kristallins. Mitteilungen der Gesellschaftder Geologie- und Sr isotopic systems. Journal of Metamorphic Geology, 26, Bergbaustudenten Österreichs, 27, 173-188.______561-581. http://dx.doi.org/10.1111/j.1525-1314.2008.00778.x_ Schermaier, A., Haunschmid, B. and Finger, F., 1997. Distribu- Tollmann, A., 1977. Geologie von Österreich. Band 1. Die Zen- tion of Variscan I- and S-type granites in the Eastern Alps. a tralalpen. 766 pp, Deuticke, Wien.______possible clue to unravel pre-Alpine basement structures. Tec- tonophysics, 272, 315-333.______Villa, I.M., 1998. Isotopic closure. Terra Nova, 10, 42-47.

Schmid, S. M., Fügenschuh, B., Kissling, E. and Schuster, R., von Blanckenburg, F. and Villa, I.M., 1988. Argon retentivity 2004. Tectonic map and overall architecture of the Alpine orogen. and argon excess in amphiboles from the garbenschists of Eclogae geologicae Helvetiae, 97, 93-117. http://dx.doi.org/ the Western Tauern Window, Eastern Alps. Contributions to 10.1007/s00015-004-1113-x Mineralogy and Petrology, 100, 1-11.______

Schmidt, W., 1921. Grauwackenzone und Tauernfenster. Jahr- von Blanckenburg, F., Villa, I.M., Baur, H., Morteani, G. and buch der Geologischen Staatsanstalt, 71, 101-116.______Steiger, R.H., 1989. Time Calibration of a PT-path from the Western Tauern Window, Eastern Alps: the problem of closure Schubert, G., Berka, R. and Philippitsch, R. 2009. Karte der temperatures. Contributions to Mineralogy and Petrology, 101, trinkbaren Tiefengrundwässer Österreichs 1:500.000 In: Ar- 1-11. beitstagung 2007 der Geologischen Bundesanstalt Blatt 67 Grünau im Almtal und Blatt 47 Ried im Innkreis: Linz, 7. - 11. Warren, C. J., Smye, A. J., Kelley, S. P. and Sherlock, S. C., Mai 2007. 2012. Using white mica 40Ar/39Ar data as a tracer for fluid flow and permeability under high-P conditions: Tauern Window, Schwinner, R., 1929. Geröllführende Schiefer und andere Eastern Alps. Journal of Metamorphic Geology, 30, 63-80. Trümmergesteine aus der Zentralzone der Ostalpen. Geolo- http://dx.doi.org/10.1111/j.1525-1314.2011.00956.x______gische Rundschau, 20, 211-244.______Wölfler, A., Kurz, W., Danisik, M. and Rabitsch, R., 2010. Da- Schwinner, R., 1951. Die Zentralzone der Ostalpen. In: X.F. ting of fault zone activity by apatite fission track and apatite Schaffer (ed.), Geologie von Osterreich, 2nd ed., pp. 105-232. (U-Th)/He thermochronometry: a case study from the Lavant- Schuster, R., Koller, F., Hoeck, V., Hoinkes, G. and Bousquet, tal fault system (Eastern Alps). Terra Nova, 22, 274-282.____ R., 2004. Explanatory notes to the map: Metamorphic struc- Wolf, R. A., Farley, K. A. and Silver, L. T., 1996. Helium diffu- ture of the Alps - Metamorphic evolution of the Eastern Alps. sion and low temperature thermochronometry of apatite. Geo- Mitteilungen der Österreichischen Mineralogischen Gesell- chimica et Cosmochimica Acta, 60, 4231-4240.______schaft, 149, 175-199.

Sölva, H., Grasemann, B., Thöni, M., Thiede, R. and Habler, G., 2005. The Schneeberg Normal Fault Zone: Normal faul- ting associated with Cretaceous SE- directed extrusion in the Eastern Alps (Italy/Austria). Tectonophysics, 401, 143-166. http://dx.doi.org/10.1016/j.tecto.2005.02.005______Received: 9 April 2014 Stüwe, K. and Schuster, R., 2010. Initiation of Subduction in Accepted: 25 August 2014 the Alps: Continent or Ocean? Geology 38, 175-178, http:// dx.doi.org/10.1130/G30528.

Thöni, M. and Jagoutz, E., 1992. Some new aspects of dating eclogites in orogenic belts: Sm-Nd, Rb-Sr and Pb-Pb isotopic 1) 1)*) 2) results from the Austroalpine Saualpe and Koralpe type-loca- Stefan PFINGSTL , Walter KURZ , Ralf SCHUSTER & 1) lity (Carinthia/Styria, SE Austria). Geochimica et Cosmochi- Christoph HAUZENBERGER ______1) mica Acta, 56, 347-368. Institut für Erdwissenschaften, Universität Graz, Heinrichstrasse 26 / Universitätsplatz 2, A- 8010 Graz; Member of NAWI Graz (an Inter Thöni, M. and Jagoutz, E., 1993. Isotopic constraints for eo-Al- University cooperation between the University of Graz and the Graz pine high-P metamorphism in the Austroalpine nappes of the University of Technology), Austria;______Eastern alps: bearing on Alpine orogenesis. Schweize-rische 2) Geologische Bundesanstalt, Neulinggasse 38, 1030 Wien, Austria;__ mineralogische und petrograpische Mitteilungen, 73, 177-189. *) Corresponding author, [email protected]______